When an optical head of an optical disk drive operates, the light emitted by a light source such as a laser diode is focused by an object lens of the optical head on an optical disk, and the light reflected by the optical disk is transmitted to a light sensor to realize information from the disk. The optical head 10 moves along two main directions, i.e. a direction perpendicular to the disk face, referred as a focusing direction F, and a direction parallel to the disk face, referred as a tracking direction T, as shown in FIG. 1.
Referring to FIG. 2, a conventional tracking control system 1 of an optical disk drive is schematically shown. The optical disk 110 is driven to rotate by a spindle motor 120. For reading/writing the rotating disk, the optical head 10 is driven to move in the tracking direction or radial direction T by a sled motor 130 to perform a seeking operation. Further, the optical head 10 is driven to move in the tracking direction T by a tracking coil 140 to perform a tracking operation. The term “tracking operation” used herein means that the position of the optical head with respect to a selected track is aligned with a proper center position of the selected track. The term “seeking operation” means that the optical pickup head jumps from one track to another track. When an electronic signal is generated responsive to the optical signal reflected from the optical disk 110 and received by the optical head 10, the electronic signal is transmitted to a radio frequency (RF) amplifier 150 to be processed into a radio frequency signal RF and a tracking error signal TE. The radio frequency signal RF and the tracking error signal TE are further processed by a digital signal processor (DSP) 170 to generate two control signals TRO and FMO. In response to the control signals FMO and TRO, a motor actuator 160 makes adjustments to output driving forces for driving the sled motor 130 and the tracking coil 140, thereby properly locating the optical pickup head 10 onto the desired track. For example, the control signal TRO facilitates tracking control of the tracking coil 140 by way of the motor actuator 160.
In general, the amplitude of the tracking error signal TE represents the tracking error amount of the optical pickup head 10. The tracking error signal TE is controlled by the closed-loop control system including the optical head 10, the radio frequency (RF) amplifier 150, the digital signal processor (DSP) 170, the motor actuator 160 and the tracking coil 140. In response to the tracking error signal TE, the control signal TRO is adjusted by the digital signal processor (DSP) 170 so as to precisely locate the optical head 10 onto the desired track.
Typically, a photo detector of an optical disk drive has main-beam light receiving parts 31 and satellite-beam light receiving parts 32 disposed at opposite sides of the main-beam light receiving parts 31. The generation of the radio frequency signal RF and the tracking error TE signal can be implemented by way of single-spot detection that detects only the light reflected from the main-beam light receiving parts 31 or triple-spot light detection that further incorporates effects of the satellite-beam light receiving parts 32.
Generally speaking, triple-spot tracking methods are advantageous over conventional single-spot tracking methods due to improved precision. Conventional single-spot tracking methods suffer from some problems. One of the examples is the offset in the tracking error signal TE that occurs in case that the spot is not perfectly aligned with the detector. Due to the alignment error Δ (also referred to as a beamlanding error), a static offset will occur in the tracking error signal (TE=L-R), wherein L and R respectively indicates signals generated by left and right portions of the main-beam light receiving parts 31, as shown in FIG. 3. The beamlanding error can be due to a misalignment of the photo detector, the position of which has always a certain tolerance and which may slightly drift in the course of time. The total static offset due to detector misalignment can easily be 200% and more. The beamlanding error caused by lens shift is another key factor resulting in the alignment error Δ. If the position of the objective lens changes, e.g. due to tracking of the disk's eccentricity, or during seeking due to acceleration and deceleration forces, the position of the spot on the detector will also change, with a factor given by the design of the light path. This results in dynamical beamlanding, and consequently in a dynamical offset, i.e. the dynamical offset in the tracking error signal TE that changes in time. The parameters of typical optical heads and media are such that this dynamical beamlanding due to lens shift cannot be ignored when using single-spot tracking methods.
Another problem that must be solved is that the system has to cope with transitions between blank and written areas, and between read and write mode. If the offset is compensated in one mode, one needs to make sure that offset compensation is still valid in another mode. For example, when the spot moves from a blank to a written area, no offset should suddenly be introduced in the tracking error signal.
Although triple-spot tracking methods are more popular than conventional single-spot tracking methods for the above reasons, there are still some limitations. For example, the triple-spot tracking methods could not be used on dual-layer BD-R/RE media due to interlayer crosstalk. The main beam returning from the other layer interferes with the satellite beams from the target layer, which causes large disturbances in the satellite signals contributing to the tracking error signal. This problem implies that for dual-layer BD media the drive should restore to single-spot tracking, where only the main beam is used as tracking information for the tracking servo loop.